1. Field of the Invention
The invention relates to a method and a device for determining the rotor resistance of an asynchronous electrical motor machine.
Modern highly dynamic four-quadrant rotating machinery such as asynchronous machines make use of a field-oriented control system. In a field-oriented control system, nominal values of the currents flowing in the stator windings assembled to a stator current vector c are preset so as to receive a predetermined angle between the vector of the normal stator current and the axis of the magnetic field of a motor using a rotating field. Using the magnetic flux vector .psi. to describe the magnetic field within the field-oriented control system and by knowing the angular position of the flux vector the field-oriented control system allows presetting of the component i.sub..psi.1 of the stator current parallel to the flux vector (magnetizing current) and the component i.sub..psi.2 perpendicular to the flux vector (active current), independently from each other, to adjust the magnetic field strength by the magnetizing current and to adjust the rotor speed and the engine torque, respectively, by the active current.
The necessary information about the magnetic flux .psi. can be obtained by subtracting the ohmic voltage drop from the phase voltages of the machine, assembled to a vector according to the assemblance of the stator current vector and by forming the stator emf-vector hereafter. The position of the flux vector of the rotor can be obtained by integrating and then subtracting the component determined by the inductive stray voltage. Obtaining the flux vector in this way, based substantially on the voltage vector, can be named "voltage model". This model frequently meets the demands for accuracy and control dynamics in the operation of the asynchronous machine, provided that the stator frequency of the asynchronous machine exceeds the rated frequency by about 10%. The integration procedure necessarily used by the voltage model, however, requires a DC contribution control, which leads, at low frequencies, to falsification of the flux determination and also impairs the control dynamics. Furthermore, to consider the ohmic voltage drop within the "voltage model" the stator resistance R.sup.s, which is temperature dependent, has to be determined. Therefore, a decrease of the stator frequency effects a discrepancy of the real temperature dependent stator resistance and the parameter describing the stator resistance within the model. That leads to falsification of the flux determination and, therefore, impairs the control system. Therefore, at lower frequencies the flux determination is provided by an arithmetic model circuit, using the stator currents and the signals pertaining to the angular rotor position to electrically simulate the flux generating events within the machine. This "current model" requires an accurate knowledge of the rotor resistance R.sup.L. This rotor resistance of an asynchronous machine is highly temperature dependent, therefore, the use of a motor resistance current model parameter value, adjusted to an average temperature-independent model parameter value, may lead to falsification. In such a way, the demand for a constant flux might not be reached and for example a defined starting moment cannot be provided by the machine. Furthermore, the falsification of the flux determination may impair the efficiency of the machine and may cause saturation phenomena.